Play behavior is important for normal social and emotional development, and the lack of play can be symptomatic in many childhood psychiatric disorders. Rats of the inbred Fischer 344 (F344) strain are consistently less playful and more anxious than the Lewis (LEW) and other strains, providing a unique opportunity to gain significant insight into the neurobehavioral control of play. This project will take advantage of te relative lack of play in F344 rats and superimpose these robust genotypic differences onto a developmental framework that allows us to manipulate early rearing experiences prior to weaning and social experiences after weaning, and assessing playfulness and anxiety during the juvenile period. Since the neuropeptide oxytocin (OT) is involved in a wide range of social behaviors, shows heritable differences in expression, and is sensitive to early experiences, this project will also begin to assess the extent to which the dysfunctional play of F344 rats is due to strain-dependent variations in OT functioning. Outcomes from this project will provide significant insights into the complex interactions between genotype and early social experiences, and how these ultimately affect behavior. As an R15 application, this project will also provide rich opportunities for undergraduates at many levels of instruction to become involved in a vibrant research program.